Everything You Need to Know. Charging your bike

Overview

Reducing Charger Voltage—A Bad Idea?

Short answer: Yes it is a bad idea, don’t lower voltage—lower amperage instead.

Voltage must remain at 42V. Lowering it tricks the BMS into thinking the battery is deteriorating. This reduces battery lifespan significantly.

Use chargers with 3A, 2A, or 1.7A instead of the original 4A charger to reduce wattage safely.

Cutting a 10s pack’s charge stop from 42.0 V to 40.0 V (4.20 → 4.00 V/cell) will help cell life short-term, but over many cycles it risks disabling balancing, corrupting state-of-charge estimation, and provoking the BMS to treat the pack as “low capacity” — producing long-term imbalance and degraded usable capacity — unless you take countermeasures.

Refined reasoning, point by point

1) BMS state estimation & adaptive behaviour

• What the BMS sees. Most BMS units estimate cell state-of-charge or “health” using either (a) direct cell-voltage thresholds, (b) coulomb-counting with periodic voltage corrections, or (c) adaptive models that compare expected cell behaviour against measured voltages.
• If the charger never reaches the designed top voltage (4.2 V/cell), the BMS never sees the expected “full” voltage profile. Over repeated cycles this can cause the BMS’s internal model to conclude the cells cannot reach 4.2 V — i.e. apparent reduced capacity/SoH. Practical results vary by BMS design:
  • Some BMSes will recalibrate downward and treat 4.00 V as the pack top (so available %SOC and charge algorithms shift).
  • Others will simply accumulate coulomb-counting error because the normal voltage-based calibration point is missing, so SOC drifts and the BMS’s idea of “100%” gets fuzzy.
• Why that’s harmful long-term: a drifting SOC or a lowered “full” threshold can hide problems (e.g., a cell that is actually weak) or cause the system to permanently accept a lower usable capacity — you lose range permanently even if cells are healthy. In some BMSes that record or report SOH, the unit may flag cells as degraded incorrectly, leading to unnecessary replacement or conservative control actions.

TL;DR: The BMS uses top-of-charge voltage as an important reference/correction. Never seeing that voltage regularly can change its algorithms and long-term behaviour.

2) Balancing requires the top voltage window

• How passive balancing usually works. Cheap and common passive balancing circuits drain small currents (~tens to a few hundred mA) from the higher cells during the CV (constant-voltage) window at the top of charge. That regime is usually something like 41.0–42.0 V pack (4.10–4.20 V/cell) where cell currents are low and the pack spends time equalising.
• If pack voltage stops at 40.0 V there is no CV top window. That means:
  • The passive balancer rarely (or never) turns on, because the pack never reaches the voltage threshold where balancing is triggered.
  • The brief low-current time where the highest cells bleed down to match the lower ones is absent.
• Result over many cycles: small initial imbalances grow. Some cells/parallel groups remain slightly higher, some lower. Over time:
  • Lowest groups get progressively undercharged (reduced usable capacity).
  • Highest groups may end up being pushed into risky voltages during any later full charge or transient, or conversely the pack as a whole loses capacity because you can’t safely use the full pack voltage spread.
• Net effect: increasing capacity loss and bigger cell-to-cell voltage spreads, which shortens useful life and can increase failure risk.

3) SOC calibration & coulomb counting

• Coulomb counting needs a calibration point. Many BMSes use coulomb counting between top-off events and use the voltage at full charge to correct accumulated error. If top-off never happens, coulomb counters drift and your BMS’s SOC readout becomes inaccurate — you’ll see unexpected cutoffs or appear to lose range faster than you should.
• Long term this leads to either false SoH readings or abrupt cutoffs when the BMS later decides cells are low or unbalanced.

4) Practical consequences for the pack and system

• Apparent permanent loss of range: The system may treat 4.0 V as “full,” so you accept less energy every charge.
• Hidden imbalance: You’ll get larger cell-to-cell spread, which can accelerate ageing of the weakest cells.
• BMS behaviour changes: Alerts, derating, charge/discharge limiting, or recorded faults may appear because the BMS’s expectations and reality diverge.
• Single occasional full charges can mask problems: If you do a full 42 V occasionally, it will rebalance and recalibrate, but skipping that step for long periods lets problems accumulate.